1. Field of the Invention
The present invention relates to a power source apparatus for an alternating-current arc welding, and more particularly, to a power source apparatus for an alternating-current arc welding which supplies an alternating-current welding current having any waveform to a load of the alternating-current arc welding by switching a current outputted from a direct-current power source.
2. Description of the Related Art
FIG. 15 shows a power source for an alternating-current arc welding which supplies an alternating-current welding current to a welding load of the alternating-current arc welding by switching an output current outputted from a direct-current power source using a switching device.
Referring to FIG. 15, a direct-current power source 101 generates a predetermined output of an alternating-current power source 102, and there is provided a switching circuit comprising switching transistors 103a to 103d and diodes 104a to 104d. The switching transistors 103a to 103d are electrically connected in a bridge form to each other, and the diodes 104a to 104d are electrically connected in parallel to respective transistors 103a to 103d, respectively, so that the conducting direction of each of the diodes 104a to 104b becomes opposite to that of each of the transistors 103a to 103d. A welding electrode 105 for an alternating-current arc welding and a work 106 are connected to the direct-current power source 101 through the switching circuit Further, there is provided a polarity switching controller 107 for controlling a pair of transistors 103a and 103b and a pair of transistors 103c and 103d so as to alternately turn on and off a pair of transistors 103a and 103b and a pair of transistors 103c and 103d.
Furthermore, there is provided a welding current setting circuit 108 for generating a reference signal Ir corresponding to a set welding current and outputting it to an error amplifier 110, and a welding current detector 109 for detecting a welding current Io flowing between the work 106 and the switching circuit and outputting a detection signal If corresponding to the welding current Io. The error amplifier 110 subtracts the detection signal If from the reference signal Ir and outputs a difference signal .DELTA.I=Ir-If between the signals Ir and If to a direct-current power source controller 111, which controls the output voltage outputted from the direct-current power source 101 according to the difference signal .DELTA.I.
In the feed back system as described above, the reference signal Ir outputted from the welding current setting circuit 108 is compared with the detection signal If outputted from the welding current detector 109, and then, the controller 111 controls the output voltage outputted from the direct-current power source 101 in such a direction that the difference signal .DELTA.I decreases toward zero so as to keep a constant output voltage outputted from the direct-current power source 101, namely, so that the welding current Io corresponds to the reference signal Ir.
On the other hand, control signals S1 and S2 are alternately supplied to either a pair of transistors 103a and 103b or a pair of transistors 103c and 103d, so that a pair of transistors 103a and 103b is turned on while a pair of transistors 103c and 103d is turned off for a time interval, and a pair of transistors 103a and 103b is turned off while a pair of transistors 103c and 103d is turned on for another time interval. When a pair of transistors 103a and 103b is turned on in response to the control signal S1, a welding current Isp having a straight polarity flows in a direction from the work 106 toward the welding electrode 105. On the other hand, when a pair of transistors 103c and 103d is turned on in response to the control signal S2, a welding current Irp having a reverse polarity flows in a direction from the welding electrode 105 toward the work 106.
FIG. 16 is a timing chart of the control signals S1 and S2 and the welding current Io.
Referring to FIG. 16, the welding current Io becomes an alternating-current of a rectangular pulse wave changing according to turning on and off of the control signals S1 and S2.
Since there is obtained a welding current of a rectangular pulse wave as described above, the welding current Io steeply changes upon switching the polarity of the voltage supplied to the welding electrode 105 and the work 106, and then, there is such an advantage that it becomes easy to perform refiring of the arc which has been extinguished once. However, since the output voltage applied to the welding electrode 105 and the work 106 becomes substantially a complete rectangular pulse wave, the arc is extinguished rapidly and the arc is reproduced rapidly upon switching the polarity, and also the change in the welding current Io is relatively large. Therefore, a large arc sounds so that an extremely large noise occurs, resulting in deterioration in the working environment. Further, since the welding current Io changes rapidly, the arc force changes rapidly. Therefore, a molten metal of the work 106 vibrates violently, and then, it is difficult for the above-mentioned power source to be applied to a precise welding.